Diagnostic and therapeutic advancements for neurological conditions will be aided by progress in biomarker research across the age span. Ultrasensitive detection assays for neurofilament light chain (NfL) have emerged as validated soluble protein measures of neuroaxonal injury.1 Abnormally elevated blood and CSF concentrations of NfL correlate with the extent of CNS injury (providing prognostic and occasionally diagnostic insights),2,3 are reduced dynamically in remission (and in association with disease-modifying treatment responses),4 and can be monitored serially to detect subclinical injury (providing an opportunity for early detection in diseases such as multiple sclerosis and perhaps a window for damage-limiting intervention in neurodegenerative, traumatic, and inflammatory CNS diseases).